The primary goal of the research in this competitive renewal application is to understand the role of signal transducers and activators of transcription (Stats), particularly Stat3, in epithelial carcinogenesis. During the previous funding period, through use of epidermis specific Stat3 knockout mice we discovered that Stat3 was required for de novo skin tumor development during two-stage carcinogenesis. Mechanistic studies revealed that Stat3 was required for both the initiation and promotion stages of skin carcinogenesis. During tumor initiation, further studies revealed that Stat3 is required for survival of DNA damaged keratinocytes, including those located in the hair follicle bulge region (i.e., stem cell compartment). During tumor promotion, Stat3 appears to be required for proliferation and clonal expansion of initiated cells. Additional studies using a new mouse model of constitutive Stat3 activation (K5.Stat3C mice) have indicated that Stat3 may also play a role in tumor progression in this model of multistage epithelial carcinogenesis. We have also found that Stat3 plays a significant role in UVB-mediated skin carcinogenesis both by protecting keratinocytes from UVB- induced DNA damage and by mediating keratinocyte proliferation following UVB exposure. Thus, Stat3 appears to play an important and similar role in both chemically-mediated and UVB-mediated skin carcinogenesis. Furthermore, we believe that Stat3 plays a particularly prominent role in keratinocyte stem cell (KSC) survival and proliferation based on additional data presented in the Progress Report/Preliminary Studies section including new data added since the previous review of this application. In the next funding period, we will examine Stat3 function specifically in bulge region KSCs and explore the role of specific Stat3 regulated genes in bulge region KSCs during epithelial carcinogenesis (both chemically-induced as well as UVB-induced carcinogenesis in mouse skin). The hypothesis to be tested is that Stat3 is required for both survival of bulge region KSCs during initiation and for proliferation and survival of initiated bulge region KSCs during promotion of epithelial carcinogenesis in mouse skin through regulation of specific genes involved in these processes. We will also test the hypothesis that Stat3 regulates specific genes that contribute to skin tumor progression in this model of epithelial multistage carcinogenesis. The specific aims are: 1) to determine the impact of Stat3 deletion and Stat3 activation on gene expression patterns, growth properties and survival of bulge region KSCs during epithelial carcinogenesis in mouse skin, 2) to examine the role of specific Stat3 regulated survival genes (i.e., Bcl-xL) in bulge region KSCs during tumor initiation by DMBA and UVB, 3) to examine the role of specific Stat3 regulated cell cycle/cell proliferation genes (i.e., cyclin D1) in bulge region KSCs during tumor promotion by TPA and UVB and 4) to further explore genes regulated by Stat3 involved in tumor progression during skin carcinogenesis. PUBLIC HEALTH RELEVANCE: Project Narrative The primary goal of the proposed research is to understand the role of signal transducers and activators of transcription (Stats), particularly Stat3 in epithelial carcinogenesis. Completion of the studies proposed in this grant application will lead to a greater understanding of the role Stat3 plays in epithelial carcinogenesis and its potential as a target for chemoprevention of cancer. Furthermore, the proposed studies will provide additional evidence that keratinocyte stem cells, particularly those found in the bulge region of hair follicles, may be the primary target cells for tumor development in mouse skin.